The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
In many applications, it is necessary to assess the wear or change of a surface relative to a base or reference. This information can be utilised for various applications, including assessing whether a surface is safe for use, or when a surface requires repair or maintenance as a result of wear over time.
In certain applications, a liner is often employed as a cost effective means of protecting a base surface from wear or damage. Consequently, the liner takes up wear in preference to the base surface, and is replaced from time to time in lieu of replacing the base surface, which may be more difficult or more expensive to replace.
Assessing the degree of wear of a surface, be it with or without a liner, is difficult or time consuming in certain environments such as where the surface is disposed internally within a cavity or compartment of a body, and especially where that body is rotatable. Conventional measurement tools are often inadequate to perform the task, either with respect to the precision of measurement, safety of performing the task, or economic factors associated with downtime of commercial use of the body whilst the measurement task is undertaken.
In some of these environments, where the comminution of material is involved and liners are used, it is important that liner wear is accurately determined to first ensure the liner is replaced before it wears to a point where it no longer protects the underlying body, and second to maintain the efficiency of the comminution process.
A practical example of the foregoing considerations is in the comminution of minerals within the mining industry. In mineral processing, minerals are extracted from their interlocked state in solid rock by crushing the raw ore into progressively smaller pieces and finally grinding it into a powder. This comminution process is multi-stage, being carried out in a series of crushing then grinding mills.
On the completion of the crushing process, the crushed ore is separated into pieces of a few cm in diameter (actual size depends on the ore type) and may then be fed into rotating cylindrical mills. The rotation of a mill about its axis causes the ore pieces to tumble under gravity, thus grinding the ore into decreasingly smaller fractions. Some types of grinding mills are fitted with grinding bodies such as iron or steel balls (ball mills), steel rods (rod mills) or flint pebbles (pebble mills) which assist in the grinding process. Two specific types of mill are the autogenous mill (AG mill), which operates without any grinding body, and the semi-autogenous mill (SAG mill), in which a small percentage (usually around 10%) of grinding bodies (often steel balls) are added.
A typical mill grinding circuit will comprise a primary grinding system, consisting of a SAG or AG mill and into which the crushed raw ore is fed, and a secondary grinding system, consisting of ball, rod or pebble mills and into which the output from the primary grinding system is fed.
All types of cylindrical mills consist of a cylindrical shell with a feed arrangement at one end and a discharge arrangement on the other. Feed and discharge designs vary. For example, feed chutes and spout feeders are common, whilst screw-type, vibrating drum and scoop-type feeders are also in use. Discharge arrangements are usually classified as overflow, peripheral, grate and open-ended.
The interior of a cylindrical mill is surfaced with a lining designed for the specific conditions of mill operation. Liners can be made of steel, iron, rubber, rubber-steel composites or ceramics. Liners in this application serve two functions:                1. to protect the shell of the mill from damage due to abrasion erosion;        2. to aid grinding performance.        
Naturally, mill liners wear through erosion. Normally, chemical solutions that are quite toxic and corrosive to humans and instrumentation alike are introduced into the mill to help with the comminution process. Whilst good liner design can enhance milling efficiency, worn liners have a detrimental effect on milling performance and energy efficiency. Therefore liners must be replaced on a regular basis.
Replacing mill liners requires significant mill downtime which is undesirable from an economic point of view. The downtime is attributable to the time taken to assess the thickness of the liner, and the considerable time needed to replace the liner. Therefore, accurately assessing the thickness of the liner within the mill is of critical importance to the mill operator. Furthermore, the minimisation of mill downtime attributable to liner thickness inspection procedures is also desirable.
One method that has been used to determine mill liner thickness is visual inspection. Once the mill has been stopped and decontaminated, a specialist enters the mill and inspects the liner for cracks, fractures and excessive wear. The problem with this approach is the time consumed in decontaminating the mill, and further, the inaccuracy of relying on the human eye to determine the thickness of an object of which the depth dimension is invisible.
Another method of determining mill liner thickness is via a physical inspection. As is the case with visual inspection, the mill must be stopped and decontaminated before the mill is inspected. A specialist enters the mill and measures the length of nails that have previously been hammered into the liner. As the liner wears faster than the protruding nail, inspection of the length of protrusion provides an indication of wear. The problem with this method is that it is time consuming in terms of mill downtime while decontamination procedures and measurement processes are executed, and further, the inaccuracy of estimating the thickness from measurements of the nail, which itself is subject to wear, against the liner wear. Further, the comparative sparsity of measurement coverage of the liner is also a problem.
Another method of determining mill liner thickness is via acoustic emission monitoring. This method involves monitoring the surface vibrations on the outside of a mill via accelerometer transducers. Estimates are obtained relating to grinding process performance and machine wear analysis. The problem with this approach is that it does not directly measure the mill liner thickness. Rather, it monitors changes in the acoustic output of a mill which could be interpreted as being due to mill liner wear, but could equally be attributable to wear of other parts of the milling machinery.
Another method of determining mill liner thickness is via ultrasonic thickness gauging. It is known by some in the industry to be a well-established technique typically performed using piezoelectric transducers. Ultrasonic gauges measure the time interval that corresponds to the passage of a very high frequency sound pulse through a test material. Sound waves generated by a transducer are coupled into the test material and reflected back from the opposite side. The gauge measures the time interval between a reference pulse and the returning echo. The velocity of sound in the test material is an essential part of the computation. The readings are obtained using a hand-held device which is operated manually within a stationary mill. The operator takes the readings by placing the sensor at selected points on the liner surface. The operator notes the thickness reading and the location on a graphical representation of the mill.
There are several problems with ultrasonic thickness gauging. Firstly, as mentioned previously, the mill must be decontaminated in order for the operator to enter the mill. Secondly, temperature alters sound velocity, and hence calibration is always needed to guarantee accurate readings. Thirdly, it is slow, as each point must be recorded manually. Fourthly, it is difficult to accurately assess liner wear due to the need to ensure that the sensor measurement tool is orthogonal to the mill shell, and the practical difficulty in achieving this.